A bear market is a sustained period of falling prices, the mirror of a bull market, typically accompanied by widespread pessimism and often economic contraction. The common informal benchmark is a decline of 20% or more from a recent high, sustained rather than a brief dip.
Bear markets tend to be shorter than bull markets on average but often feel more intense, since sharp, fast declines are common within them alongside the possibility of sudden strong rallies ("bear market rallies") that don't end the broader downtrend. Distinguishing a genuine trend reversal from a temporary rally within a bear market is one of the harder calls in investing, and is rarely obvious until well after the fact.
As with "bull market," the term applies at any scale — a single stock, a sector, or an entire market can be described as being in a bear market independently of the others.